1. Field of the Invention
The present invention relates to a method of measuring a resistance of a magnetoresistive (MR) element such as a read head employed in a magnetic recording medium drive including a hard disk drive (HDD), for example.
2. Description of the Prior Art
In general, a magnetoresistive (MR) head element has the maximum permissible magnitude of a supplied electric current. The maximum permissible magnitude can be determined depending on the magnitude of the resistivity of the MR head element. If an electric current of a magnitude beyond the maximum permissible magnitude is supplied to the MR head element, the lifetime of the MR head element is remarkably shortened. Accordingly, the MR head element is only allowed to receive a sense or bias current of a magnitude below the maximum permissible magnitude.
When the maximum permissible magnitude of an electric current should be specified for MR head elements, the resistance is estimated for the individual MR head elements. The resistance can be estimated based on the value of an inspection current supplied to an MR head element and the potential difference or voltage appearing at opposite terminals of the MR head element in response to supply of the inspection current. The value of the inspection current may be set at a predetermined level at a current source for the inspection current. The potential difference or voltage can be read as the output of the MR head element when the MR head element receives the inspection current.
If the estimated resistance is actually smaller than the true or effective resistance of the MR head element, the calculated maximum permissible magnitude of an electric current is specified as a magnitude larger than the true maximum permissible magnitude. If the sense current is set based on the thus incorrect maximum permissible magnitude, the MR head element should suffer from a shortened lifetime. Heretofore, the resistance of an MR head element cannot be measured at a higher accuracy. A predetermined margin is added to the calculated maximum permissible magnitude in view of the extent of such an error, when the magnitude of a sense or bias current should be determined. The set magnitude of the sense current sometimes largely deviates from the true maximum permissible magnitude.
It is a trend in the technical field of magnetic recording media such as magnetic disks to increase or improve the recording density. A higher linear density along recording tracks induces reduction in the magnitude of the output signal from the MR head element. A larger sense current is required to still enlarge the magnitude of the output signal. If the sense current is too small, the signal/noise (S/N) ratio is supposed to get deteriorated in the output from the MR head element, so that a binary data recorded on the medium cannot correctly be decoded into its original information data. Measurement of the resistance at a higher accuracy may serve to cancel the deviation between the calculated maximum permissible magnitude and the true maximum permissible magnitude for an electric current. The magnitude of a sense current can be maximized within the range below the true maximum permissible magnitude of an electric current.
It is accordingly an object of the present invention to provide a method of measuring the resistance of a magnetoresistive (MR) element at a higher accuracy.
According to a first aspect of the present invention, there is provided a method of measuring the resistance of an MR element, comprising determining the amplification characteristic of an amplifier designed to amplify the potential difference appearing at a sense channel prior to calculation of the value corresponding to the resistance of the MR element connected to the sense channel.
In general, the potential difference or voltage appearing at terminals of the MR element is detected after amplification at an amplifier. It is impossible to set the amplification characteristic or gain at a higher accuracy in the amplifier designed to amplify a smaller and weaker potential difference or voltage. The effective gain of the amplifier slightly departs from the ideal gain actually expected. The determination of the resistance based on the actual measurement of the amplification characteristic leads to calculation of the value corresponding to the resistance of the MR element at a higher accuracy.
According to a second aspect of the present invention, there is provided a method of measuring the resistance of an MR element, comprising: supplying an inspection current to a resistor connected to a sense channel; measuring the value of an output voltage from an amplifier designed to amplify the potential difference appearing at the sense channel in response to supply of the inspection current; and calculating the amplification characteristic of the amplifier based on the value of the output voltage and the value of the inspection current.
In general, the amplification characteristic or gain of an amplifier may be specified by the ratio of the output voltage to the input voltage. The value of the input voltage can be obtained by calculation based on the values of the resistance of the resistor and the inspection current, as conventionally known. On the other hand, the value of the output voltage can be measured. Based on the calculated input voltage and the measured output voltage, it is possible to obtain the amplification characteristic or gain of the amplifier in a facilitated manner.
In addition, the effective resistance can be set in a resistor, such as a wire-wound resistor, at a higher accuracy. The resistor usually exhibits the effective resistance of the value similar to the indicated value of the specifications. It is accordingly possible to determine the effective resistance of the resistor at a higher accuracy without actual measurement or inspection. The indicated value of the specifications is useful enough to determine the amplification characteristic of an amplifier at a higher accuracy.
When the effective resistance of an MR element is to be measured, an inspection current may be supplied to the MR element connected to the aforementioned amplifier through a sense channel. In this case, the value is measured for a voltage output from the amplifier based on supply of the inspection current. The values of the inspection current and the measured output voltage in addition to the amplification characteristic or gain enables a highly accurate determination of the value corresponding to the resistance of the MR element. Without measurement of the gain for the individual amplifier, it is impossible to determine the resistance of the MR element at a higher accuracy.
Furthermore, according to a third aspect of the present invention, there is provided a method of measuring the resistance of an MR element, comprising: supplying a first inspection current of a first value to a resistor connected to a sense channel; measuring a first value of an output voltage from an amplifier designed to amplify the potential difference appearing at the sense channel in response to supply of the first inspection current; supplying a second inspection current of a second value different from the first value to the resistor; and measuring a second value of an output voltage from the amplifier in response to supply of the second inspection current.
The potential difference or voltage expected to appear at terminals of the resistor in response to supply of the first and second inspection currents can be calculated based on the value corresponding to the resistance of the resistor as well as the first and second values of the inspection currents. The amplification characteristic or gain of the amplifier can be calculated in a facilitated manner based on the potential difference appearing at the resistor and the measured first and second values of the outputs from the amplifier. In addition, employment of the resistor enables a highly accurate measurement of the amplification characteristic or gain for the amplifier, in the aforementioned manner.
When the effective resistance of an MR element is to be measured, an inspection current of a third value may be supplied to the MR element which is connected to the aforementioned amplifier through a sense channel. In this case, the value is measured for a voltage output from the amplifier based on supply of the inspection current. The third value of the inspection current and the value of the measured output voltage in addition to the amplification characteristic or gain enables a highly accurate determination of the value corresponding to the resistance of the MR element. Without measurement of the gain for the individual amplifier, it is impossible to determine the resistance of the MR element at a higher accuracy.
The method of measuring may be accomplished by a read operation controller circuit comprising: a plurality of sense channels; a resistor connected to one of the sense channels; a current source designed to output an electric current of a specified value; and a bias circuit connected to the current source and designed to appoint any of the sense channels so as to supply the electric current from the current source to said any of the sense channels.
For example, the current source may be designed to output the aforementioned first and second inspection currents of the first and second values. The first and second inspection currents can be supplied from the current source to the resistor via the bias circuit and the specific sense channel. In response to supply of the first and second inspection currents, the first and second values can be measured for the output voltage from the amplifier, respectively. The amplification characteristic or gain can be determined in accordance with the first and second values of the inspection currents and the measured first and second values of the output voltages, in the aforementioned manner.
The individual MR elements can be connected to the corresponding sense channels in the read operation controller circuit. When an inspection current is supplied to the respective sense channels, the voltage of the output from the amplifier can be measured for the individual MR elements. The obtained values of the inspection currents and the measured voltages can be employed to accurately determine the effective resistance of the respective MR elements.
Furthermore, the method of measuring may be accomplished by a magnetic recording medium drive comprising: a magnetoresistive element; a first sense channel connected to the magnetoresistive element; a resistor; a second sense channel connected to the resistor; a current source designed to output an electric current of a specified value; a bias circuit connected to the first and second sense channels so as to selectively supply the electric current from the current source to the first and second sense channels; and an amplifier connected to the first and second sense channels so as to amplify a potential difference appearing at the first and second sense channels in response to supply of the electric current from the current source.
For example, the current source may be designed to output the aforementioned first and second inspection currents of the first and second values as well as the inspection current of the third value. The first and second inspection currents can be supplied to the resistor from the bias circuit through the second sense channel. On the other hand, the inspection current of the third value can be supplied to the MR element from the bias circuit through the first sense channel. The output voltage from the respective sense channels can be measured in this manner in response to supply of the inspection currents of the first, second and third values. The amplification characteristic or gain can be determined in accordance with the first and second values of the inspection currents and the measured first and second values of the output voltages, while the effective resistance of the MR element can be determined at a higher accuracy, in the aforementioned manner.
Furthermore, the method of measuring can be achieved by the operations of a computer. In this case, a recording medium may include a computer applicable software allowing the computer to process: appointing a sense channel connected to a resistor; causing supply of a first inspection current of a first value to the sense channel; obtaining a first value of an output voltage from an amplifier designed to amplify a potential difference appearing at the sense channel in response to supply of the first inspection current; causing supply of a second inspection current of a second value different from the first value to the sense channel; obtaining a second value of an output voltage from the amplifier in response to supply of the second inspection current; and calculating an amplification characteristic of the amplifier based on a value corresponding to a resistance of the resistor, the first and second values of the first and second inspection currents and the first and second values of the output voltages. The recording medium may include a flash memory mounted on a printed circuit board, for example, a flexible disk (FD), a compact disk (CD), and the like.